Recently, enlargements in the scale of shipping vessels, building construction, offshore structures, and the like, have been undertaken, in order to add value thereto. Since such structures and vessels may cause catastrophic environmental damage, fatalities, and property loss in a single accident, types of steel having high degrees of strength, extra levels of thickness and high low-temperature toughness need to be used in the construction of such structures and vessels.
For the efficient development of such steels, suitable welding operations are required, and the most widely used welding technique in the case of welding such steels is the flux-cored arc welding (FCAW) technique.
The securing of stability in welded structures obtained from the welding technique described above is an important consideration, and in this regard, securing tensile strength and impact toughness in welded joints is essential. In addition, in order to prevent low-temperature cracks in a welded joint during a flux-cored arc welding operation, decreasing the content of diffusible hydrogen within welding materials is crucial.
Generally, a welded joint formed during a welding operation may form a coarse columnar structure when a portion of steel is diluted to form a melt pool while welding materials are melted, and the melt pool is subsequently solidified. The structure thereof may be changed according to the welding materials and an amount of heat input during welding. Since coarse grain boundary ferrite, Widmanstätten ferrite, martensite, and martensite austenite (M-A) constituents may be formed in coarse austenite grain boundaries of a welded joint, impact toughness thereof may be significantly deteriorated.
Therefore, in the case of the metals of offshore structures and the like, refinements in the metal structures thereof have been pursued through the complex addition of alloying elements, such as nickel (Ni), titanium (Ti), boron (B), and the like, along with the addition of deoxidizing, denitrifying, or dehydrogenating elements, in order to secure low-temperature impact toughness.
A mechanism for structural refinement by the complex addition of Ti—B—Ni may generate fine ferrite in austenite grains through matrix toughening by Ni, inhibitive action of pro-eutectoid ferrite formation due to prior austenite grain boundary segregation of dissolved B, and Ti, B, oxides and nitrides.
As described above, there is a need to secure impact toughness in welded joints by controlling the microstructure of welded joints in order to secure stability of a welded structure.
In the related art with respect to the above, Patent Document 1 provides technology relating to a technique regulating compositions and microstructures of welded joint, which describes an ultra high-strength SAW welded joint equal to or greater than 950 MPa grade, having excellent low-temperature toughness containing 0.7 to 0.8 wt % of carbon, a microstructure of a welding metal portion containing 10-20% of low bainite and martensite, and 60 area % or more of acicular ferrite.
In addition, Patent Document 2 and Patent Document 3 relate to an ultra high-strength steel pipe having seam welding joint with excellent cold-crack resistance and manufacturing methods thereof. Here, excellent cold-crack resistance may be secured through the inclusion of 1% or more retained austenite in the seam welding metal portion, but impact toughness of the welding metal portion may be deficient.
While Patent Document 4 defines components of welding materials, since such components do not directly control the microstructure, particle size, and the like of a welded joint, it may be difficult to obtain a sufficient degree of welded joint toughness from such welding materials.
On the other hand, in order to prevent the formation of low-temperature cracks in high-strength welded joints, a content of diffusible hydrogen may be maintained to be as low as possible.
In the case of a rutile-based flux-cored wire according to the related art, a diffusible hydrogen content of a welded zone is within a degree of 8-10 ml/100 g. However, when welding thick, high-strength steel using such flux-cored wire, a preheating operation is required for the prevention of low-temperature cracks, and an additional operating charge may arise as a result.
Generally, a rutile-based flux-cored wire is used in a flux-cored arc welding technique, which is manufactured by drawing a wire such that a diameter of the wire has a size suitable for welding after filling the wire with flux containing high amounts of crystal water and bound water which serve as resources for diffusible hydrogen during a welding operation. However, in the wire drawing process, adhered and remaining organic components of lubricants may increase the diffusible hydrogen content of a welded zone during the welding operation.
In order to resolve the problem as described above, Patent Document 5 proposes a technique relating to a high-temperature heat treatment of a tubular wire at 600-800° C. However, the commercialization of such a technique may be difficult due to a decreased manufacturing speed caused by a high-temperature heat treatment and an increased high heat treatment costs.
In addition, Patent Document 6 provides a preheat-free flux-cored wire for 490 Mpa or more high tensile steel, containing 0.5% to 4.5% of an arc stabilizer and a slag former, and 1.0% to 4.0% of a deoxidizer, based on 0.05% to 0.25% of vanadium (V), and describes diffusible hydrogen trapping through VC formation by the addition of V and a carbon fixation effect. However, it may be difficult to secure stable arc properties since the effect of the diffusible hydrogen reduction result by V during welding may be insignificant, and the total amount of a fluorine (F) content of alkali and an alkaline earth metal-based fluoride is high, for example, 1.0% to 2.0%.
Patent Document 7 describes a technology providing a flux, containing 4.0% to 8.0% of titanium dioxide (TiO2), 0.02% to 0.4% of alkali metal fluoride (fluorine equivalence), and 0.02% to 0.4% of polytetrafluoroethylene (PTFE) (fluorine equivalence) based on the total weight of the wire; regulating the fluorine equivalence of alkali-earth metal fluoride to 0.01% or less; and controlling (fluorine equivalence of alkali metal fluoride+0.35)/(fluorine equivalence of PTFE) to be 1 or more. However, it may be difficult to commercialize such a wire due to a low specific gravity of PTFE compared to that of fluoride present in a mineral state causing wire filling and surface application to be difficult, and due to costs being relatively high.
Therefore, there is a need for the development of welding materials for an improvement in impact toughness of a welded joint, as well as a reduction diffusible hydrogen during welding.
(D1) Korean Patent Laid-Open Publication No. 2009-0016854
(D2) Japanese Patent Laid-Open Publication No. 2000-256779
(D3) Japanese Patent Laid-Open Publication No. 2002-115032
(D4) Japanese Patent Laid-Open Publication No. 11-170085
(D5) Korean Patent Laid-Open Publication No. 1998-0068561
(D6) Japanese Patent Laid-Open Publication No. 1996-257785
(D7) Korean Patent Laid-Open Publication No. 2007-0035996